1. Field of the Invention
The present invention relates to a method of treating a gas sensor element. More particularly, this invention relates to such a method of treating a gas sensor element for improving measuring characteristics of the gas sensor element arranged to measure a gas component having bound oxygen such as NOx, SOx, etc., in a measurement gas.
2. Discussion of Related Art
As a sensing device for measuring NOx, SOx, etc., in combustion gases and exhaust gases of internal combustion engines, there are conventionally used a NOx sensor, a SOx sensor, and the like which include a gas sensor element for measuring a gas component (measurement gas component) having bound oxygen such as NOx or SOx, contained in a gas to be measured (measurement gas). The gas sensor such as the NOx sensor or the SOx sensor has various configurations. As one example, there is known a gas sensor constituted by including a gas sensor element which has an electrochemical cell provided by forming, on a solid electrolyte, a measuring electrode capable of reducing or decomposing the measurement gas component having the bound oxygen in the measurement gas and formed of a cermet of a noble metal material and a ceramic material.
In such a sensor element of the gas sensor constructed as described above, the NOx sensor, for instance, is constituted by including an oxygen ion-conductive solid electrolyte body 10 such as ZrO2. As shown in FIG. 1 schematically indicating an internal structure of the sensor element, there are formed within the solid electrolyte body 10: a first internal space 14 which is located on a distal side of the solid electrolyte body 10 for communication with an exterior through a first diffusion controlling section 12; a second internal space 18 which communicates with the first internal space 14 through a second diffusion controlling section 16; and a reference-air introducing passage 20 which is open at a proximal end of the solid electrolyte body 10 for communication with an atmospheric air. In the thus constructed sensor element, a measurement gas existing outside the solid electrolyte body 10 is introduced into the first internal space 14 through the first diffusion controlling section 12 under a predetermined diffusion resistance, and an atmosphere (the measurement gas) in the first internal space 14 is introduced into the second internal space 18 through the second diffusion controlling section 16 under a predetermined diffusion resistance, while a reference air is introduced into the reference air-introducing passage 20 through the opening at the proximal end of the solid electrolyte body 10.
In the thus constructed NOx sensor element, a first solid electrolyte portion 22 of the solid electrolyte body 10 in which the first internal space 14 is formed, an inner pumping electrode 24 formed on a portion of the first solid electrolyte portion 22 which is exposed to the internal space 14, and an outer pumping electrode 26 formed on an outer surface portion of the first solid electrolyte portion 22 which is exposed to the exterior of the solid electrolyte body 10 cooperate with one another to constitute an electrochemical cell functioning as a main pumping cell 28. Further, a second solid electrolyte portion 30 interposed between the second internal space 18 and the reference air-introducing passage 20, a measuring electrode 32 formed on a portion of the second solid electrolyte portion 30 which is exposed to the second internal space 18, and a reference electrode 34 formed on a portion of the second solid electrolyte portion 30 which is exposed to the reference air-introducing passage 20 cooperate with one another to constitute an electrochemical cell functioning as a measuring pumping cell 36. Moreover, the first solid electrolyte portion 22, the second solid electrolyte portion 30, the inner pumping electrode 24, and the reference electrode 34 cooperate with one another to constitute an electrochemical pumping cell functioning as an oxygen partial pressure-detecting cell 38. In FIG. 1, a reference numeral 40 indicates a heater for heating the NOx sensor element.
In the NOx sensor element constructed as described above, a desired voltage is applied between the two electrodes 24, 26 of the main pumping cell 28 by using a variable power source (not shown) to allow a current to flow in a predetermined direction, so that oxygen in the atmosphere (the measurement gas) of the first internal space 14 is pumped out into an external measurement gas-existing space, or, on the contrary, oxygen is pumped into the first internal space 14 from the external measurement gas-existing space. An electromotive force induced between the two electrodes 24, 34 of the oxygen partial pressure-detecting cell 38 based on a difference in an oxygen concentration between the atmosphere in the first internal space 14 and the reference air in the reference air-introducing passage 20 is measured by a suitable potentiometer (not shown). Further, a desired voltage is applied between the two electrodes 32, 34 of the measuring pumping cell 36 by using a variable-voltage power source (not shown), so that oxygen in an atmosphere (the measurement gas) of the second internal space 18 is pumped out into the reference air-introducing passage 20. The measuring electrode 32 of the measuring pumping cell 36 is formed of a porous cermet composed of a noble metal material such as rhodium (Rh) capable of reducing or decomposing NOx, and a ceramic material such as zirconia (ZrO2). The measuring electrode 32 functions as a catalyst for reducing or decomposing NOx.
In the NOx sensor element constructed as described above, oxygen is pumped into or out from the first internal space 14 by the oxygen pumping action of the main pumping cell 28, and an oxygen partial pressure in the atmosphere of the first internal space 14 is controlled to be a predetermined, desirably low value that does not allow reduction of NOx, by controlling the voltage of the variable power source to be applied between the two electrodes 24, 26 on the basis of a detected value of oxygen partial pressure in the atmosphere of the first internal space 14 detected by the oxygen partial pressure-detecting cell 38. The atmosphere of the first internal space 14 with the oxygen partial pressure being controlled is introduced into the second internal space 18 via the second diffusion controlling section 16, and NOx in the atmosphere is reduced in the second internal space 18 by the measuring electrode 32 functioning as the catalyst for reducing or decomposing NOx. Oxygen generated upon reduction of NOx is pumped out into the reference air-introducing passage 20 from the second internal space 18 by the oxygen pumping action of the measuring pumping cell 36. In this instance, since the oxygen partial pressure (the oxygen concentration) in the atmosphere of the first internal space 14 is controlled to be constant, a pumping current which is proportional to the concentration of NOx is allowed to flow between the measuring electrode 32 and the reference electrode 34 of the measuring pumping cell 36.
Accordingly, in the conventional NOx sensor element described above, the pumping current in the measuring pumping cell is measured, and the concentration of NOx in the measurement gas is obtained based on the measured pumping current.
For examining detecting sensitivity to the gas of the conventional gas sensor elements having the structure described above, the inventors of the present invention performed various experiments using the NOx sensor element, and it was found that the conventional gas sensor elements suffered from the following problems.
A NOx sensor element which has the structure described above and which is not used yet for measurement after it has been produced was actually installed on a vehicle, and a durability-evaluating test was performed on the NOx sensor element. The result of the test revealed the following: the NOx sensor element suffered from initial deterioration in its NOx detecting sensitivity, that is, the NOx sensor element suffered from about 20%-reduction in the NOx detecting sensitivity at a time point where about 50 hours passed after the moment of initiation of the measurement, and thereafter the detecting sensitivity was stabilized. Further, the concentration of NOx in a measurement gas without containing NOx was measured using the unused NOx sensor element which is not used yet for measurement. Since NOx is not contained in the measurement gas, the pumping current should be zero, and accordingly the NOx concentration should be zero. However, the pumping current of about 0.8 μA was detected, and the NOx concentration corresponding to the detected pumping current was measured. Thus, the NOx sensor element suffered from initial offset in the NOx detecting sensitivity.
Accordingly, it was revealed that, in the conventional gas sensor elements, the initial deterioration and initial offset of the detecting sensitivity to the measurement gas component are inevitably caused, adversely influencing the measurement of the measurement gas component in the measurement gas.